A typical ignition device of an internal combustion engine includes a spark plug disposed in a combustion chamber, an ignition coil that supplies energy on combustion to the spark plug, and an ECU (Electronic Control Unit) that controls ignition timing of the ignition coil.
For such an ignition device, it has been proposed to detect an ion current generated after combustion in the combustion chamber. According to a combustion state detected based on the ion current, it is possible to grasp an operation state of the internal combustion engine, or to realize various electronic controls such as lean combustion.
In general, an apparatus for detecting an ion current includes a capacitor that holds an electric charge caused by a current from the spark plug, a Zener diode that limits a voltage of the capacitor, and a load resistor that detects the ion current (Patent Documents 1 and 2).
Patent Document 1: Japanese Unexamined Patent Publication No. 04-194367
Patent Document 2: Japanese Unexamined Patent Publication No. 11-037031